Fully automated diagnostic analyzers are commercially available to perform chemical assays and immunoassays of biological fluids such as urine, blood serum, plasma, cerebrospinal liquids and the like. Generally, reactions between an analyte in a patient sample and reagents used during the assay generate a signal from which the concentration of analyte in the patient sample may be calculated. Such automated analyzers generally use an aspirating means such as a sampling tip, or probe or needle, to transfer desired volumes of liquid samples or liquid reagents between receptacles, such as between sample containers, reagent containers and reaction cuvettes disposed on the analyzer. Hereinafter, variations of the term aspirate refer to all of such processes for extracting liquid from one container and depositing at least some of the liquid into the same or another container and further includes the supporting devices required to complete the liquid handling operations.
Aspirators typically comprise an elongated, needle-like probe or pipette having a hollow passage whereby liquid may be aspirated into and/or dispensed from the probe using appropriate pumping resources. The pipette may be carried by a transport mechanism adapted to provide horizontal and vertical movement so as to enable the pipette tip to be lowered into a liquid in a reservoir for aspiration of the liquid, and for transporting the liquid to a another location whereat the pipette is lowered to an optimal position for dispensing the liquid. Some type of device, such as a piston assembly, which may be incorporated into the pipette, is operated electronically to aspirate liquid into the pipette and to dispense liquid from the pipette using vacuum pressures.
It is desirable, when aspirating a liquid, to quickly and accurately determine if the amount of liquid available for aspiration is smaller than the desired amount of aspirated liquid; that is, if air is aspirated into the probe during an aspiration process. Various methods are known for detecting such a “short sample” situation as described below.
U.S. Pat. No. 6,938,504 discloses a liquid dosing process in which a temporal course of at least one state variable of a medium is determined essentially over the entire duration of the dosing process and is mathematically compared with a pre-determined state variable nominal range by correlation.
U.S. Pat. No. 6,370,942 discloses a method for determining aspiration of air by employing three separate aspiration tests including a pressure difference test to verify liquid was aspirated. Three algorithms are employed using data acquired throughout the aspiration process and each must produce a positive result for the sample to be released for transfer elsewhere.
U. S. published patent application 20070143063, incorporated herein by reference, discloses and a method for verifying the integrity of an aspiration process by determining the profile of an entire aspiration pressure curve and then determining by numerical analysis whether the difference between the actual and the mathematical is less than the standard deviation of the residuals of a linear regression analysis of an aspiration pressure curve measured on a sample known to have clots therein or known to be less than a desired aspiration volume.
In the short sample detection schemes described above, the shape of the aspiration curve is generally analyzed over a fairly large portion of the full aspiration process. Typically, these systems depend on measuring vacuum pressure at different pre-determined intervals throughout the aspiration process and comparing calculated values to a range of predetermined satisfactory values. However, as the state of the art in clinical analyzers advances, the amount of operational data processed by an analyzer's controller has increased dramatically, placing a premium on the amount of time during which operational situations like a short sample are detected as well as reducing the amount of data required to be reported while monitoring the analyzer's operational status. Hence, there is a need for a more efficient method for determining the insufficiency of sample during a liquid aspiration process, in particular a method that requires less data acquisition than required in the known art.